SpudFiles

But it will be very hard for me to find one as I am in South Africa, so I think I will have to stick to ball valve as its available.

uhhh ebay http://cgi.ebay.com/ASCO-8353C33-Dust-C ... .m20.l1116

they are unconventional on this site, but theres all ways the good old 10 pound solid steel(maybe iron) butterfly valves, but they come at a price....

Burst disk valve anyone? Means you don't need a safety valve either...

Oh and there's several more South Africans on here, some don't frequent very often anymore.

MrCrowley wrote:Burst disk valve anyone? Means you don't need a safety valve either...

Also means you have no idea when the gun is actually going to fire. That, IMO, is the height of stupidity.

jimmy101 wrote:

MrCrowley wrote:Burst disk valve anyone? Means you don't need a safety valve either...

Also means you have no idea when the gun is actually going to fire. That, IMO, is the height of stupidity.

Pretty much the same with most burst disk guns, you have little control of when it will fire within a certain degree.

This is easily avoidable by possibly two things;

1: Do multiple tests to get a reasonable idea of when the cannon will fire. Don't take it too seriously but use it more of a guide. IE, I wouldn't stand in front of the cannon even though tests I had previously done shows that it will explode several minutes from now as there are a lot of contributing factors that can alter that time frame.

2: Be prepared. Make sure no one is in front of the cannon, never go in front of the cannon if the burst disk hasn't burst, make sure the cannon is secure and that no one will accidently walk in front of the cannon.

Depending on your burst disks and the dry ice, most cannons will probably fire in under 2 minutes. That's not much waiting around and you can go and take cover somewhere.

I don't see the problem with that. Though I would recommend loading the projectile first, yes it might accidently fire with the projectile when you're not ready, but you can take precautions to make sure you setup the cannon where a misfire wont be much of a problem. Better than filling the chamber then trying to load the projectile anyway.

The burst disk idea is good its just the problem of the gun firing when you dont really know. I might consider it, but right now I just want to make things as simple and cheap as possible and get it built, so im using a ball valve for now. But I might choose to do a few upgrades further on in time . I would like to post a design of it soon so you could all see.

MrCrowley wrote:Pretty much the same with most burst disk guns, you have little control of when it will fire within a certain degree.

Not any of the burst disk guns I've ever seen. Since burst disks are mostly used with combustions then there is minimal pressure on the disk until the fuel is ignited. Yes there is some variability it how long after ignition the burst disk will fail but that is usually measured in fractions of seconds.

The hole idea of a dry ice powered burst disk gun gives me the willys.

jimmy101 wrote: Since burst disks are mostly used with combustions.
The hole idea of a dry ice powered burst disk gun gives me the willys.

There are plenty of pneumatic burst disk cannons also, which I guess is what i'm referring to as like you said, with a combustion you pretty much know when the burst disk will burst.

I've personally tried the burst disk dry ice cannon before. Just a small hand-held, metal chambered cannon I made just to see how well it would work. As long as you're sensible, they're pretty safe. Install a ball valve just in case you need to relieve the pressure also.

MrCrowley wrote:Pretty much the same with most burst disk guns, you have little control of when it will fire within a certain degree.

Only if the person building it is an idiot.

Burst discs are easy to control (and I know you know that!).

D_Hall wrote:

MrCrowley wrote:Pretty much the same with most burst disk guns, you have little control of when it will fire within a certain degree.

Only if the person building it is an idiot.

Burst discs are easy to control (and I know you know that!).

Assuming you've done proper testing, yes. I did say they were controlable to a degree, i.e the burst disk wont burst at say 90PSI every single time, it might burst at a little higher or lower pressure. If you're using an air compressor and just flooding the chamber with 120PSI, you'll have no worries, but a track pump is a different story and there could be a few seconds where you expect it to burst but it hasn't or it bursts a tad early.


Just trying to say that a dry ice burst disk cannon isn't such a bad idea assuming you're not an idiot and have done the required testing with disk burst pressure, dry ice quantities (and water if you use it) and timed test fires. When I buy dry ice it comes in small cylinders about 3cm long and 1cm in diameter. Wouldn't be too hard to calculate how many you need to put in your chamber (with possibly water) to make a 90PSI burst disk burst and how long it would roughly take.

I would use the same safety procedures with a dry ice cannon as I would with any other cannon.

MrCrowley wrote:Assuming you've done proper testing, yes. I did say they were controlable to a degree, i.e the burst disk wont burst at say 90PSI every single time, it might burst at a little higher or lower pressure. If you're using an air compressor and just flooding the chamber with 120PSI, you'll have no worries, but a track pump is a different story and there could be a few seconds where you expect it to burst but it hasn't or it bursts a tad early.

I get the feeling you're being deliberately obtuse.

Cascading systems using either positive or negative pressure fluctuations.
Mechanically or pyrotechnically triggered systems.
Heck, there was somebody around here working one in which a nichrome wire would melt (read: weaken) a plastic disc on command.

In any event, you CAN have very precise control of a burst disc cannon.

When I buy dry ice it comes in small cylinders about 3cm long and 1cm in diameter.

Total aside, but that's interesting. 'Round here it comes in 20# bricks.

....obtuse. ...
..

There it is again, the dorkiest word in modern history that I can think of.

Besides it reminds me of the guy (movie) who got b*tt r*ped repeatedly,
should've killed his wife anyway (but didn't cuz he was a punk anyhow)....

...And stared with that anoying blaeck dewd...he s*cks in real life probablt. 8) [/quote]

D_Hall wrote:

MrCrowley wrote:Assuming you've done proper testing, yes. I did say they were controlable to a degree, i.e the burst disk wont burst at say 90PSI every single time, it might burst at a little higher or lower pressure. If you're using an air compressor and just flooding the chamber with 120PSI, you'll have no worries, but a track pump is a different story and there could be a few seconds where you expect it to burst but it hasn't or it bursts a tad early.

I get the feeling you're being deliberately obtuse.

Cascading systems using either positive or negative pressure fluctuations.
Mechanically or pyrotechnically triggered systems.
Heck, there was somebody around here working one in which a nichrome wire would melt (read: weaken) a plastic disc on command.

In any event, you CAN have very precise control of a burst disc cannon.

When I buy dry ice it comes in small cylinders about 3cm long and 1cm in diameter.

Total aside, but that's interesting. 'Round here it comes in 20# bricks.

Whether or not you can control a burst disk isn't the issue. The issue is a noob using dry ice in a gun. Burst disk or not it isn't a good idea.

Dry Ice comes in several common forms. ~12" square blocks, ~12x12x2" slabs and pellets. When used with a liquid as a cooling bath the pellets are by far the most effective. In most usages the pellets cool faster than the blocks since they have much greater surface area. The slabs and blocks are usually used without a liquid medium in things like transport cases.

In chem labs, ethanol and acetone are are generally used as the liquid phase in a dry ice cooling bath. An acetone + dry ice bath will chill small objects to ~-78C (~-108F) in a couple seconds. A hunk of dry ice all by itself is a pretty inefficient and slow way to cool something.

D_Hall wrote: In any event, you CAN have very precise control of a burst disc cannon.

True, but I think a downside of triggered burst disk guns is that they can give people a false sense of security. With a ball valve, it is EXTREMELY unlikely the gun will fire when you don't want it to (piston and other piloted valves are obvioulsy a bit more prone to firing without the user intending to, as any leak behind the piston could trigger it.) With a triggered burst disk, flaws in the disk or in the mechanism clamping it could all contribute to a premature firing. Of coruse, with proper precautions it doesn't matter either way, but it's still somthing people building triggered disks should be aware of at all times.

The other downside is that you have to replace the disk after each shot, which can get annoying.